SO ANATOMY OF THE HEAD AND NECK. 



another. It covers the encephalon loosely, and leaves 

 considerable space between it and the convolutions; thi 

 space, called the sub-arachnoid, is greatest at the base 

 the brain, and here as elsewhere contains more or less fluid, 

 which, from the continuity existing between the snb-aracl 

 noid space of the cranium and that of the spinal canal, 

 called the cerebro-spinal fluid. The cerebro-spinal fluid 

 however, often lost in the course of dissection, from wound- 

 ing a dependent portion of the membrane, through whi< 

 it drains off from both the cerebral and spinal spaces. 



The PIA MATER is the vascular covering of the brain, 

 and this, likewise, envelops both the encephalon and spins 

 cord ; it is in close contact with the cerebral substance, an< 

 dips into the fissures and convolutions ; it also penetrates 

 into the interior of the brain, to supply vessels to the walls 

 of the cavities there existing. 



In connection with the membranes will be found th< 

 arteries of the brain, derived from the vertebral and inter- 

 nal carotid arteries. 



The vertebral artery (p. 58) supplies the cerebellum an< 

 posterior lobes of the cerebrum ; it ascends by the side of 

 the medulla oblongata, and at the posterior border of the 

 poris Yarolii unites with its fellow to form the basilar artery. 

 Prior to their union the}' give off certain small branches 

 distributed to the spinal cord and its membranes, as well 

 as the posterior meningeal artery to the falx cerebelli and 

 cerebellar fossae. 



The inferior cerebellar arteries are derived from the 

 vertebrals near the point of their union, or from the 

 basilar artery ; they wind around the upper part of the 

 medulla oblongata, and are distributed to the under sur- 

 face of the cerebellum, their branches penetrating th< 

 fissure between its hemispheres. 



The BASILAR ARTERY is the vessel resulting from the 

 union of the two vertebrals ; it reaches from the posterioi 

 to the anterior border of the pons Varolii, and terminates 

 by dividing into two branches for the cerebrum ; it gives 

 off numerous small transverse branches, the most anterior 

 of which are called the superior cerebellar arteries ; these 

 wind around the cms cerebri on each side, and are dis- 

 tributed to the upper surface of the cerebellum, anastomos- 

 ing with the inferior cerebellar arteries. 



The posterior cerebral arteries are the terminal branches 

 of the basilar; they are of large size, and are directec 



